In an electric power grid, power consumption and generation must be constantly controlled in order to maintain both network frequency and voltage within the permitted limits. The distribution network constantly monitors the two magnitudes and must be capable of sending orders to the generating units in order to stabilise the electric power of the grid. As a consequence of this requirement, the power-generating units must be capable of regulating the power, whether primary or secondary. Understanding primary regulation to be the generation of power equal to that assigned, plus a value proportional to frequency deviation, to react to frequency variations with a time constant in seconds, and understanding secondary regulation to be that which is responsible for restoring the frequency in minutes. Therefore, when an imbalance occurs in an area, initially, all the power stations will contribute to compensate it.
In recent years, the power generated by wind farms has increased significantly worldwide. Wind farms are growing in size and installed capacity, and the importance of improving both the delivered power quality and grid stability will be a challenge of great importance to both wind farm developers and the electricity distribution network. As wind-power generated is injected into the network, care must be taken to ensure that wind farm behaviour is as similar as possible to other conventional power generation sources, taking the specific nature of the wind into account.
As mentioned earlier, from the viewpoint of the company that owns the distribution network, it is important to accurately control network voltage and frequency. To this end, conventional power generation plants must deliver an extra supply of active power when required by the grid. At present, wind farms do not adequately meet these types of demands, deriving in the restriction of wind farm expansion. To date, a wind farm cannot be considered as a conventional power generation plant due to the impossibility of accurately determining an active power reserve due to the specific nature of the wind. Contrary to the power sources of conventional power generation plants (coal, oil, gas, uranium, etc.), wind is an uncontrolled and highly unpredictable resource. In fact, conventional methods have not been capable of accurately estimating the active power reserve or accurately meeting power demands. Consequently, there is an evident need to develop an efficient method for generating an active power reserve whenever the grid requires it. In fact, it must be highlighted that on some occasions advantageous economic conditions can even be offered to those capable of satisfying network demands in terms of active power reserve.
Patent EP1282774 (Aloys Wobben) proposes a method by means of which the turbine reduces the active power delivered to the electric grid when network frequency increases. Patent US2007085343 (Jens Fortmann, Repower System AG) includes a method for providing extra power if frequency variations are detected in order to participate in primary regulation, although in a transitory manner. Finally, EP1467463 (Lütze, Hans Henning, et al., General Electric Company), claims a method for controlling the power delivered by a wind farm based on network frequency.
The document by Ramtharan, Support for spinning reserve from DFIG-based wind turbines, proposes working with a low-torque set point (or even zero) and regulating the machine with the pitch at maximum speed. When it receives an order from the operator it changes its torque reference, taking the machine to its optimal point. This technique does not guarantee a quantified reserve as the available power reserve depends on wind speed at all times.
None of the aforementioned documents allows or guarantee the amount of available power as an active power reserve, nor do they manage it under the conditions required by electric grid operators.
The invention proposed has several advantages over current systems for controlling active power and frequency in wind farms.
One of the advantages of the present invention is that it achieves an active power reserve under the conditions required by grid operators from conventional power generation plants. An active power reserve with respect to producible power at a given time for each of the machines is ensured, thereby achieving an active power reserve at wind farm level.
An additional advantage of this method is that it allows a bidirectional variation in active power, i.e. it can achieve a gradual increase or decrease in active power. Additionally, this active power reserve is controlled by sufficient dynamics to guarantee primary or secondary regulation across the whole range of machine power outputs.
Another advantage is its independence with respect to the wind farm's communications network, as each of the turbines is equipped with an internal control, in such a manner that frequency variations are detected by the wind turbine itself, ensuring the reaction of the controller even in the event that communication with the farm is interrupted.
Another additional advantage is that frequency-reserve regulation does not compromise machine speed regulation at any time, as it interacts with the turbine's speed controllers at all times.
In conclusion, the present invention has a method that is capable of guaranteeing primary and secondary regulation. Said regulation allows wind farm behaviour to be as similar as possible to that of conventional power generation sources, collaborating in network frequency stabilisation and contributing to increase the penetration of renewable energies in the electric grid.